Reduction of capping layer resistance area product for magnetic device applications

ABSTRACT

A ferromagnetic layer is capped with a metallic oxide (or nitride) layer that provides a perpendicular-to-plane magnetic anisotropy to the layer. The surface of the ferromagnetic layer is treated with a plasma to prevent diffusion of oxygen (or nitrogen) into the layer interior. An exemplary metallic oxide layer is formed as a layer of metallic Mg that is plasma treated to reduce its grain size and enhance the diffusivity of oxygen into its interior. Then the plasma treated Mg layer is naturally oxidized and, optionally, is again plasma treated to reduce its thickness and remove the oxygen rich upper surface.

RELATED PATENT APPLICATION

This application is related to Ser. No. 12/931,866, filing date Feb. 11, 2011 and to Ser. No. 13/068,172 filing date May 4, 2011, assigned to the same assignee as the current invention and fully incorporated herein by reference.

BACKGROUND

1. Technical Field

This disclosure relates generally to magnetic devices that utilize thin film magnetic layers with out-of-plane magnetic anisotropy, and more specifically, to methods for reducing the resistance-area product of individual layers without reducing the overall magnetoresistance of the device.

2. Description of the Related Art

Many present day magnetic devices utilize thin films with an “out-of-plane” (i.e., perpendicular to the film plane) magnetization direction, which is often referred to as perpendicular magnetic anisotropy (PMA). Such devices include, but are not limited to,

(1) various designs of magnetic random access memory (MRAM), e.g., PMA (or Partial-PMA) Spin-Torque MRAM in which such films can serve as pinned layers, reference layers, free layers, or dipole (offset-compensation) layers;

(2) various designs of PMA spin valves, tunnel valves (magnetic tunnel junctions—MTJs) and PMA media used in magnetic sensors and magnetic data storage, and;

(3) other Spintronic devices.

For these applications it is often important to employ PMA films with a large coercivity field, Hc, and anisotropy field, Hk. It is also important to reduce, as much as possible, the resistance-area product (RA) of the individual layers, high values of which reduce the overall magnetoresistance (MR) of the structure. A variety of methods have been employed to achieve this result, with various degrees of success. Examples can be found in:

-   U.S. Patent Application 2011/0086439 (Choi). -   U.S. Pat. No. 7,750,421 (Horng et al.). -   U.S. Pat. No. 7,602,033 (Zhao et al.). -   U.S. Patent Application 2009/0162698 (Fukuzawa et al).

SUMMARY

A first object of the present disclosure is to provide such a reduction of the RA product of a thin-film magnetic device, while, at the same time, providing out-of-plane magnetic anisotropy to the magnetic layers.

A second object of the present disclosure is to provide such a PMA device with a high thermal stability.

A third object of the present disclosure is to provide such a PMA device where the free layer has enhanced values of Hc and Hk.

A fourth object of the present disclosure is to fulfill the previous objects while improving the design compatibility of the device and its method of fabrication with the design and fabrication requirements suitable for applications within the area of high-density, PMA spin-torque transfer MRAM.

The present disclosure achieves these objects by means of a capping layer and its method of formation. This capping layer, which is a thin, insulating layer of magnesium oxide (MgO), will provide out-of-plane anisotropy to the free layer (or other ferromagnetic layer) of a device on which it is formed. Note that the general effect of certain oxides in promoting vertical magnetic anisotropies of ferromagnetic thin films is described in related patent application Ser. No. 12/931,866, which is fully incorporated herein by reference.

In addition, the present capping layer will provide high thermal stability, low resistance-area product (RA) and improved compatibility with the design and processing requirements of such magnetic devices. The method of this invention and the capping layer it produces is, therefore, highly suitable for applications in, e.g., the manufacture of high-density PMA Spin-Torque MRAM, and other spintronic based inventions as specified in the objects above.

According to the present method, the capping layer is a thin insulating magnesium oxide layer that will provide additional perpendicular anisotropy to the free layer. The capping layer is formed by the deposition and oxidation of a thin magnesium layer. Since this magnesium oxide layer contributes to the resistance of the stack, it reduces the overall MR of the structure.

A common approach to reducing RA of an oxide layer is to under-oxidize it. Unfortunately, the perpendicular anisotropy induced at the interface between the free layer and the capping layer is directly proportional to the oxidation state of the interface, which makes it difficult to reduce the RA of the oxide layer without also reducing Hk.

The present method provides a plasma treatment of a deposited (on a free layer) metallic magnesium layer before it is oxidized, which allows a reduction in the amount of oxygen necessary to oxidize the layer at the interface with the free layer. Therefore, the RA of the oxidized layer is reduced while preserving or even improving the induced perpendicular anisotropy of the free layer on which it is formed.

Plasma treatment can also be performed after the oxidation of the MgO, thus reducing the thickness of the oxide layer and also reducing the RA. Additionally, some plasma, treatment performed on top of the free layer also reduces the RA which is due, we believe, to the improved smoothness at the interface.

Typical previous methods of forming the oxidized cap layer produce a rough interface between the free layer and the oxide layer which allows oxygen to diffuse into the free layer. As illustrated schematically in the sequence FIGS. 1 (a), (b), (c), (d), the resulting oxidation profile (oxygen concentration as a function of penetration into a layer) is not abrupt at the interface.

FIG. 1 (a) shows a schematic vertical cross-section of an Mg metal layer (10) deposited on a ferromagnetic material layer (20) (CoFeB, written as “CFB” in this and following figures). Note that the interface (23) is shown as a wavy line to indicate a non-smoothed surface.

After oxidation, as represented schematically by the arrows (30) in FIG. 1 (b), the Mg metallic layer has been converted into an oxidized Mg layer (40), denoted MgO hereinafter. A thin layer of oxidized CoFeB (50) (written as (CFB)Ox hereinafter) is also formed between interfaces (24) and (25) between the oxidized Mg and the unoxidized CoFeB (20).

The highly schematic graph in FIG. 1 (d) shows a dashed line (100) representing the rather non-uniform oxygen concentration profile as measured downward from the MgO (40) surface being subjected to the oxidation process.

Finally, FIG. 1 (c) shows the same layers of FIG. 1 (b) subsequent to an annealing process that smooths the oxygen distribution (now shown as a solid line (110) in FIG. 1 (d)) in the capping layer, leading to a somewhat flatter distribution. Yet, the oxidation of the cap layer (40) needs to be high in order to achieve PMA since it is mostly the oxygen concentration at the interface (24) with the free layer which determines the induced perpendicular anisotropy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 (a), (b), (c) and (d) are schematic illustrations, including a graph, of a prior art method of oxidizing a Mg layer, formed as a metal layer on a ferromagnetic layer, such that the oxygen concentration profile does not achieve the objects of the present invention.

FIGS. 2 (a), (b), (c), (d) and (e) schematically show an improved series of steps, according to an embodiment of the present invention, in which a ferromagnetic layer is smoothed by a plasma treatment before deposition of a metallic Mg layer, which is then oxidized.

FIGS. 3 (a), (b), (c), (d), (e) and (f) schematically show additional improvement of the steps of FIGS. 2 (a)-2 (e) by adding a step of plasma treatment of the oxidized Mg layer.

FIGS. 4 (a), (b), (c), (d), (e) and (f) schematically show additional improvement of the steps of FIGS. 3 (a)-3 (f) by adding a second step of plasma treatment to the ferromagnetic layer before formation of the oxidized Mg layer.

FIGS. 5(a), 5(b), 5(c), 5(d) and 5(e) describe a fabrication otherwise similar to FIGS. 3(a)-3(f), except a native oxide of the CoFeB free layer is used instead of an oxide of an Mg layer.

FIGS. 6(a), 6(b) and 6(c) each show an exemplary Polar Kerr hysteresis map corresponding, respectively, to a particular fabrication illustrated in rows 1, 3 and 4 of the table in FIG. 6(d) to verify that the fabrication has perpendicular magnetic anisotropy.

FIG. 6(d) shows a table indicating the magnetic and resistive properties of several fabrications as a result of different capping formations and different plasma treatments.

DETAILED DESCRIPTION

Preferred embodiments of the present disclosure provide a series of improvements over the previous method illustrated in FIGS. 1 (a)-1 (c). The improvements will be seen in terms of an improved oxygen concentration profile of the MgO and (exemplary) CoFeB layers. It is noted that the process to be described herein could also be applied using oxides of elements chosen from the group consisting of Si, Ba, Ca, La, Mn, Zn, Hf, Ta, Ti, B, Cu, Cr, V and Al. In addition, the process to be described below could be carried out using nitrides of the same group of elements, if a nitride capping layer is to be used in place of an oxide layer.

Referring next to FIG. 2 (a), there is shown schematically a first aspect of a preferred embodiment of the present disclosure, which is to smooth the (exemplary) CoFeB free layer (20) surface (22) with a plasma treatment (as indicated schematically by the arrows (200)) before the metal deposition on that surface.

Referring next to FIG. 2(b) there is now shown, schematically, the deposition of a metallic layer of Mg (10) on the now smooth, interfacial surface (27). The resulting smooth interface (27) between the Mg layer and the exemplary CoFeB layer will be less prone to diffusion of oxygen, making the resulting oxidation profile more abrupt at the free layer/cap layer interface. The oxidation profile will be shown in the graph of FIG. 2(e), discussed below.

Referring next to FIG. 2(c), there is shown an oxidation process (schematically represented by arrows (300)) applied to the Mg layer (10) to form a layer of MgO (15). The smooth interface (27), however, serves to act as a barrier to diffusion of the oxygen into the body of the CoFeB (written as CFB) free layer.

Referring next to FIG. 2 (d), there is shown the MgO layer (15) formed on the CoFeB layer after both the completion of the oxidation process in FIG. 2(c) of the Mg layer and the completion of an annealing process to improve the PMA properties of Hc and Hk.

Referring finally to the graph in FIG. 2(e), there are shown two oxygen concentration profiles. A dashed curve (100) represents the oxygen profile before the anneal referred to in FIG. 2(d), while the solid curve (110) represents the oxygen concentration profile after the anneal. The more discontinuous nature (evidenced by a sharper reduction to zero concentration) of the after-anneal profile (110) at the interface would allow for higher interfacial anisotropy generated at the interface and lower RA contribution from the oxidized free layer. Still, a high oxidation of the cap layer is necessary to ensure good PMA.

The disclosed cap layer structure illustrated in these and the following figures (all of FIGS. 2, 3 and 4) can be deposited at room temperature or at elevated temperature (up to 400° C.). The additional annealing step (shown in FIG. 2(d), FIG. 3(e) and FIG. 4(e)) of up to 400° C., with ˜300° C. preferred, must be used to improve the PMA properties, such as increasing the Hc and Hk. The effect of the anneal is to produce the required anisotropy by modifying the interface between the ferromagnetic material and the oxide.

The most beneficial plasma treatment is made after the deposition of the metal cap layer, but before its oxidation, which will be shown in detail in FIGS. 4(a)-4(f). This results in an improved oxidation of the interface between the free layer and the capping layer which increases the amount of induced perpendicular anisotropy at the free layer interface. A plasma treatment that achieves the objects of the disclosure and is used hereinafter, utilizes a radio-frequency generated plasma at low power (i.e., examples of between approximately 10 w and 35 w are described in FIG. 6) in the presence of an inert gas such as Ar, Kr or Xe.

The material of the cap layer can in theory be any oxide or nitride material which provides perpendicular anisotropy at the interface with the free layer which contributes to the RA of the full structure thus reducing the MR. Such materials include but are not restricted to: Al oxide, Mg/Ta Oxide, Ti oxide, Zn oxide, oxides of Si, Ba, Ca, La, Mn, Hf, B, Cu, Cr, V or native CFB oxide, or to nitrides of Mg, Si, Ba, Ca, La, Mn, Zn, Hf, Ta, Ti, B, Cu, Cr, V and Al.

Referring next to schematic FIGS. 3(a), 3(b), 3(c), 3(d), 3(e) and 3(f), there is shown schematically a second aspect of a preferred embodiment which provides a further improvement to the capping layer and enhances its effect on the free layer. This aspect of the disclosure is to etch back the formed oxide to remove the highly oxidized top portion of the MgO before anneal, as will be shown in FIG. 3 (d).

Referring first to FIGS. 3(a)-3(c), there are shown the same steps as previously illustrated in FIGS. 2(a)-2(c), during which a layer of metallic Mg (10) is deposited on a smooth CoFeB underlayer (20) and then oxidized (FIG. 3 (c) (15)).

Referring now to FIG. 3(d), there is shown an additional step in which a plasma treatment (arrows (400)) is applied to the upper surface of the MgO layer, which, before the plasma treatment, is both rough and over-oxidized. This plasma application removes (etches-back) an upper portion of the MgO layer, smoothing the surface of the remaining portion and thinning the remaining portion, while removing an over-oxidized portion. The reduced thickness of the remaining portion of the oxide layer and the removal of the over-oxidized portion helps in reducing the RA without sacrificing the PMA properties of the structure. Referring to the oxygen concentration profile of FIG. 3(f), there is shown that the removal of the oxygen-rich upper portion of the MgO layer has truncated the oxygen concentration profile (110).

Referring now to schematic FIGS. 4(a), 4(b), 4(c), 4(d), 4(e) and 4(f), there is shown yet another aspect of the preferred embodiment in which the metallic Mg layer is treated with plasma before it is oxidized to both smooth the Mg layer and reduce its grain size.

Referring first to FIG. 4(a), there is shown the preliminary plasma treatment (arrows) (200) of the surface of the exemplary CoFeB layer, as also done in FIGS. 3(a) and 2(a).

Referring next to FIG. 4(b) there is shown schematically the deposition of a metallic Mg layer (111), but the layer is now shown as a quasi-granular substance having large grains. This granular structure was also present in the deposited Mg layer of FIGS. 3(b) and 2(b), but it was not shown there, as no further changes in grain size was to occur in the remainder of those processes.

Referring next to schematic FIG. 4(c), there is shown the effect of a plasma treatment (arrows (250)) on the grain size and thickness of the deposited Mg metallic layer (120). Specifically, the grain size is reduced and the metallic layer is smoothed, rendered more continuous in its internal structure and reduced in thickness.

Referring next to FIG. 4(d), there is shown an oxidation process (arrows (500)) applied to the metallic Mg layer of reduced grain size ((120) in FIG. 4 (c)) to produce a layer of MgO (150). The reduced grain size renders this oxidation process more effective as the oxygen diffuses into the body of the layer with a greater uniformity of concentration.

Finally, FIG. 4(e) shows the fabrication of FIG. 4(d) subsequent to an anneal as described in relation to the previous figures.

Referring now to the graph of oxygen concentration shown in FIG. 4(f), there is shown the concentration profile both before the anneal (dashed curve (100)) and after the anneal (solid line curve (110)). These two curves, which are smooth and quite similar, illustrate that when a plasma treatment is performed on the metallic layer before oxidation (FIG. 4(c)) the resulting oxygen distribution profile is much improved as a result of more effective oxygen diffusion around the smaller grains of the metal. Thus, the effect of the plasma treatment is to both smooth and thin down the metallic layer and provide a smooth continuous metallic layer before oxidation. Another benefit is the reduction of the grain size. This reduction allows for more uniform oxidation of the metallic layer as the grain boundaries help the diffusion of the oxygen atom through the layer. For a given target oxidation of the interface, the oxidization step can be reduced greatly compared to the prior art.

We have also noted above that the process steps illustrated in FIGS. 4(a)-4(e) can be carried out using the native oxide of the ferromagnetic free layer, which here has been an exemplary layer of CoFeB (or another alloy of Co, Fe and B).

Referring now to the sequence of steps in FIGS. 5(a), (b), (c) and (d), the fabrication of FIG. 5(a) shows a layer of ferromagnetic material (20), such as the exemplary CoFeB (CFB), which is used as a free layer.

Referring to FIG. 5(b), there is shown the CoFeB layer (20) being subjected to a plasma treatment (arrows (200)), which smooths the free layer and may also change the grain size of the free layer.

Referring next to FIG. 5(c), there is shown the smoothed free layer of 5(b) now being subjected to an oxidation process (arrows (500)) which forms a native oxide layer (60) on the CoFeB layer.

Referring next to FIG. 5(d), there is shown the oxidized CoFeB layer being subjected to a second plasma treatment (arrows (250)), which smooths and reduces the thickness of the previously formed oxide layer (65). Finally, referring to FIG. 5(e), there is shown the formation of an optional capping layer of metallic Mg (70) over the smoothed oxide layer (65).

To illustrate the benefits of the different plasma treatment approaches, several exemplary structures have been built on an MgO barrier layer (i.e., a tunneling barrier layer such as might be found in an MTJ structure) as shown in the table of FIG. 6(d). Polar Kerr maps of hysteresis curves displayed in FIGS. 6(a), 6(b) and 6(c) substantiate that the structures of rows 1 (6(a)), 3 (6(b)) and 4 (6(c)) are, indeed, PMA with enhanced properties that meet the objects of the disclosure. These particular structures could be used quite advantageously to form the free layer of an MTJ device in a spin torque transfer configuration.

Row #1 of the table in FIG. 6(d) shows (in column labeled FL) that a free layer is first formed as two layers of CoFeB of thickness 10 angstroms, separated by a Ta layer 1.5 angstroms in thickness. The column labeled “FL Cap” indicates the capping structure formed on the free layer as well as the particular oxidation and plasma treatments (PT, measured in watts, seconds) applied to that structure. The FL Cap of row 1 consists of a 4.5 angstrom thick layer of metallic Mg, oxidized by natural oxidation (Nox) using an oxygen flow rate of 18 sccm for 300 seconds, followed by a further deposition of 2.5 angstroms of Mg. A Ru capping layer is applied (column labeled Cap), followed by an anneal at 300° C. for 10 minutes (column labeled Anneal). The RA product is measured at the layer center as 13.28 ohm-micron².

The table of FIG. 6(d) also shows that applying a plasma treatment to the surface of the metallic Mg or to the surface of MgO (with slightly more power due to the lower etch rate of the MgO) is effective in reducing the R. It also shows that using a plasma treatment on top of the free layer also reduces RA.

In summary, the disclosed structure offers:

1. Improved PMA properties in a ferromagnetic layer in terms of its Hk and Hc,

2. Improved compatibility of such a layer and its fabrication method with high-temperature processing, and,

3. Improved RA.

These properties make the process and structure suitable for a variety of applications, particularly for use in advanced PMA Spin-Torque MRAM devices. The structure and process can also be utilized in those in-plane magnetic devices (i.e., whose magnetization is aligned in plane of the films) in which it is beneficial to introduce an out-of-plane component of magnetic anisotropy, such as in the In-Plane Spin Torque MRAM devices or Partial-PMA Spin Torque MRAM devices.

As is finally understood by a person skilled in the art, the preferred embodiments of the present disclosure are illustrative of the present disclosure rather than limiting of the present disclosure. Revisions and modifications may be made to methods, materials, structures and dimensions employed in forming and providing a capped ferromagnetic layer structure having a component of perpendicular-to-plane magnetic anisotropy enhanced and maintained by plasma treatments of the capped and capping layers, while still forming and providing such a structure and its method of formation in accord with the spirit and scope of the present invention as defined by the appended claims. 

What is claimed is:
 1. A capped ferromagnetic layer having perpendicular-to-plane magnetic anisotropy, comprising: a ferromagnetic layer having an upper surface in a plane of its deposition; wherein a first plasma treatment has been applied to smooth said upper surface and make it resistant to oxygen or nitrogen diffusion; and wherein an oxide layer or a nitride layer is formed as a cap on said plasma-treated upper surface to promote a perpendicular-to-plane interfacial magnetic anisotropy in said ferromagnetic layer; and wherein a region extending from an upper surface of said oxide layer or said nitride layer to an interface with said upper surface of said ferromagnetic layer is characterized by an oxygen concentration profile or a nitrogen concentration profile; and wherein an annealing process has been applied to said capped ferromagnetic layer to sharply reduce said oxygen or said nitrogen profile to approximately zero at said upper surface interface with said ferromagnetic layer and thereby provide higher perpendicular-to-plane interfacial magnetic anisotropy as evidenced by an enhanced Hk and Hc and, in addition, to provide an improved value of a resistance and surface area product, RA.
 2. The capped ferromagnetic layer of claim 1 wherein an upper surface of said oxide layer or said nitride layer has been etched back, thinned and made smooth by a third plasma treatment thereby removing an over-oxidized or over-nitrided upper portion and eliminating a source of excess oxygen or nitrogen that would be adversely distributed during said anneal.
 3. The capped ferromagnetic layer of claim 1 wherein said oxygen concentration profile or said nitrogen concentration profile is smoothly reduced from its maximum value at an upper surface of said oxide layer or said nitride layer, respectively, to an approximately zero value at an interface between said oxide layer or said nitride layer, respectively, and said ferromagnetic layer.
 4. The capped ferromagnetic layer of claim 1 wherein said ferromagnetic layer is a free magnetic layer.
 5. The capped ferromagnetic layer of claim 1 wherein said cap is an oxide layer that is a native oxide layer of the ferromagnetic material.
 6. The capped ferromagnetic layer of claim 5 wherein an upper surface of said native oxide layer has been made smooth by a third plasma treatment.
 7. The capped ferromagnetic layer of claim 5 wherein said native oxide layer has been thinned by a third plasma treatment.
 8. The capped ferromagnetic layer of claim 7 where a metal layer of Mg is formed over said thinned oxide layer.
 9. The capped ferromagnetic layer of claim 1 wherein said ferromagnetic layer is a tri-layer comprising two layers of CoFeB of approximately 10 angstroms thickness separated by a layer of Ta of approximately 1.5 angstroms thickness.
 10. The capped ferromagnetic layer of claim 9 wherein said capping layer is an oxide layer that is a first metallic layer of Mg of thickness approximately 6 angstroms that is oxidized and plasma treated and over which a second layer of metallic Mg of thickness 3 angstroms is formed.
 11. The capped ferromagnetic layer of claim 1 wherein said cap is an oxide layer that is a deposited metallic layer that is subsequently oxidized.
 12. The capped ferromagnetic layer of claim 11 including a second plasma treatment applied to an upper surface of said deposited metal layer prior to said oxidation process whereby a grain size of said deposited metal layer is reduced, resulting in a reduced metallic grain size, and a uniform oxidation is subsequently achieved.
 13. The capped ferromagnetic layer of claim 12 wherein said deposited metal layer is a layer of Mg.
 14. The capped ferromagnetic layer of claim 12 wherein said deposited metal layer is a layer of Si, Ba, Ca, La, Mn, Zn, Hf, Ta, Ti, B, Cu, Cr, V or Al.
 15. The capped ferromagnetic layer of claim 12 wherein said second plasma treatment produces a smooth upper surface of said deposited metal layer.
 16. The capped ferromagnetic layer of claim 12 wherein said reduced metallic grain size promotes a more uniform oxygen diffusion through said metal layer and thereby produces a more uniform oxide layer.
 17. A TMJ spin torque transfer cell comprising: a reference layer having a perpendicular-to-plane component of magnetic anisotropy; a tunneling barrier layer formed on said reference layer; a free layer having a perpendicular-to-plane component of magnetic anisotropy; a capping layer formed as a metallic oxide or nitride layer on said free layer by the oxidation or nitrogenation of a metal layer first deposited on an upper surface of said free layer, wherein said capping layer maintains a perpendicular-to-plane component of magnetic anisotropy of said free layer while providing a low value of RA, by means of a combined sequence of plasma treatments that, in a first plasma treatment, smooths said upper surface of said free layer to prevent diffusion of oxygen or nitrogen from said capping layer into said free layer, then, in a second plasma treatment by reducing the grain size of said first deposited metal layer, then oxidizing said metal layer and then, in a third plasma treatment removing an overoxidized or overnitrogenized portion of said oxidized or nitrogenized metal layer and, finally, by a magnetic anneal that produces an oxygen or nitrogen concentration profile in said capping layer that goes to zero at the interface of said free layer. 